The present invention relates to a breaker composition for oil fracking and to its use in oil fracking.

The use of chemical breakers in oil fracking is known from prior art, namely from US 8415278, which explains the main principles behind the technique summarized below.

Hydrocarbons (e.g., oil and natural gas) reservoirs can be reached by drilling a wellbore into the earth. Such a wellbore can be used to directly extract hydrocarbons or as an injector well to inject a fluid, e.g., water or gas, to drive the relevant fluids/gasses into a production wellbore.

In this field, a treatment performed to enhance or restore the productivity of a well is called a stimulation treatment. Stimulation treatments fall into two main groups, matrix treatments and hydraulic fracturing treatments.

In general, hydraulic fracturing involves injecting a fracturing fluid through the wellbore.

The fracturing fluid is injected through the wellbore at such a high flow rate and under such high pressure that the rock of the subterranean formation that is subjected to the hydraulic treatment literally cracks apart or fractures under the strain, which provides an easy path for the oil to easily flow into the well.

However, once the high pressure is relieved by the escape of the fracturing fluid, the fracture has a tendency to be squeezed closed by the natural pressures on the rock within the deep subterranean formation. To keep the fracture open, some kind of material (a“proppant”, generally sand or gravel) must be placed in the fracture to prop the faces of the fracture apart.

To be able to carry and place a proppant into a newly-created fracture, a fluid must have a sufficient viscosity to suspend and carry the proppant.

Typically, the fracturing fluid is a gel, which has high viscosity and enough capacity for carrying a proppant suspended in the fluid. Typically, it is a polymer gel. Common gelling polymers include, for example, guar, guar derivatives, xanthan, chitosan, starch, starch derivatives, cellulose and cellulose derivatives. For example, one of the most common gelling polymers used in the oil industry is crosslinked guar. After the proppant is mixed with the viscous fracturing fluid and pumped downhole to form a fracture, the fracturing fluid must be removed from the formation. It should be removed without moving the proppant from the fracture and without damaging the conductivity of the proppant bed. This is generally done by thinning the viscous fluid that transported the proppant to a very low viscosity near that of water for optimal removal from the propped fracture. Reducing the viscosity of a viscosified fluid is referred to as "breaking" the fluid and the chemicals used to reduce the viscosity of fracturing fluids are called breakers.

Chemical breakers used to reduce viscosity of natural gelling polymers used in fracturing or other subterranean applications such as guar and derivatized guar polymers are generally grouped into three classes: oxidizers, enzymes, and acids. All of these materials reduce the viscosity of the gel by breaking the polymer chain. The breakers operate by cleaving the backbone of polymer either by hydrolysis of acetal group, cleavage of glycosidic bonds, oxidative/reductive cleavage, free radical breakage or combination of these processes. A breaker should be selected based on its performance in the temperature, pH, time, and desired viscosity profile for each specific treatment.

Oxidizers commonly used to reduce viscosity of natural polymers includes, for example, sodium persulfate, potassium persulfate, ammonium persulfate, lithium and/or sodium hypochlorites, chlorites, peroxide sources (sodium perborate, sodium percarbonate, calcium percarbonate, urea-hydrogen peroxide, hydrogen peroxide, etc.), bromates, periodates, permanganates, etc.

The idea behind the present invention is to use an imido- alkanpercarboxylic acid as oxidizer breaker. More specific breakers of the invention are phthalimido-peracetic acid, e-phthalimido peroxyhexanoic acid, 3- phthalimido-perpropionic acid, 4-phthalimido-perbutyric acid, 2-phthalimido- diperglutaric acid, 2-phthalimido-dipersuccinic acid, 3-phthalimido-perbutyric acid, 2-phthalimido-perpropionic acid, 3-phthalimido-diperadipic acid, naphthalimido-peracetic acid, 2-phthalimido-monopersuccinic acid. Especially e- phthalimido peroxyhexanoic acid (or PAP) gives good results within the frame of the invention.

Therefore, the present invention relates to the use of an imido- alkanpercarboxylic acid in a breaker composition for oil fracking. Preferably, the imido-alkanpercarboxylic acid is e-phthalimido peroxyhexanoic acid (or PAP). This compound is commercialized by Solvay under the brand Eureco™. Below it will be referred to PAP for sake of conciseness but it should be understood that the present invention also relates to the other imido-alkanpercarboxylic acid mentioned above.

The advantages of using this type of oxidizer instead of the other peroxides mentioned above are the following:

• Safety: unlike most of the peroxides PAP is to be regarded as a safe chemical. Its formulations are stable and do not undergo self-heating phenomena or bring to self-decomposition risks.

• Low temperature activation: PAP is a preformed peracid, therefore it does not need to be activated. Even at room temperature PAP exploits its performance with acute effects.

• High electrochemical oxidation potential: though very stable, PAP shows a high potential of oxidation so much that it can be used in chemical synthesis in place of highly hazardous chemicals like meta-chloro perbenzoic acid. · High performance: the high oxidation potential allows using minor amounts of PAP in different applications to achieve an optimal performance.

• Suitability: PAP is an organic chemical, as a consequence it is possible to obtain clear liquid organic solutions to bring its reactivity where needed. The organic solutions are stable for a suitable period of time.

Therefore, in a preferred embodiment, the breaker composition comprises an organic solvent, preferably a solvent such that the breaker composition remains liquid in freezing conditions, for instance at least down to -40°C.

Solvents that give good results in that regard are alcohols, namely IPA

(IsoPropyl Alcohol), MEG (MonoEthylene Glycol), TEG (TriEthylene Glycol) or PG (Propylene Glycol). IPA gives good results in practice.

In another preferred embodiment, the oxidizer breaker of the invention is used in combination with another breaker, for instance of the acid type. It is namely so that the oxidizer breaker of the invention acts rather rapidly so that in order to have a prolonged action it might be advantageous to add a breaker with a slower action ln that regard, it has been found that dibasic esters and more precisely, dicarboxylic dimethylesters like the mixture of Dimethyl Glutarate, Dimethyl Succinate and Dimethyl Adipate commercialized under the brand Rhodiasolv® RPDE, give good results. They namely hydrolyse on contact with water with resultant acid which breaks the gel. Hence, in this preferred embodiment, the breaker composition comprises an acid breaker or a precursor thereof like an ester, preferably a dibasic ester, more preferably a dicarboxylic dimethylester, most preferably a mixture of dicarboxylic dimethylesters. In practice, a mixture of Dimethyl Glutarate, Dimethyl Succinate and Dimethyl Adipate gives good results.

Typical amounts of the above mentioned ingredients are the following: 1 ) PAP 2-6 grams optionally stabilized with HEDP ( 1 -hydroxyethylidene- 1,1- diphosphonic acid)

2) Rhodiasolv RDPE 4-15 grams

3) IPA or MEG or TEG 80 grams

Such formulations gave good results in terms of stability at room temperature and visual break of a crosslinked gel at 60°C over 24 hours, dosage lml/L.

The present invention also relates to a breaker composition comprising an imido-alkanpercarboxylic, an organic solvent and an organic acid or a precursor thereof like an ester. Preferably, said imido-alkanpercarboxylic acid is PAP.

In a preferred embodiment, the organic solvent is an alcohol, preferably

IPA.

ln another preferred embodiment, the breaker composition comprises at least one dibasic ester as organic acid precursor, preferably a mixture of dicarboxylic dimethylesters.

More preferably, this mixture of dicarboxylic dimethylesters is a mixture of Dimethyl Glutarate, Dimethyl Succinate and Dimethyl Adipate.

The present invention also relates to the use of the above described breaker composition in oil fracking.